Variable frequency oscillator



- vJ1me 12, 1945. K. RATH 2,378,245

VARIABLE FREQUENCY OSCILLATOR Filed March 17, 1942 2 Sheets-Sheet 2 to IF "amplifier IN VEN TOR.

Patented June 12, 1945 VARIABLE FREQUENCY OSCILLATOR,

Karl Rath, New York, N. Y., assignor to-Radio Patents Corporation, NewYork, N. Y., a corporation of New York Application March 17, 1942,Serial No. 435,037

6 Claims.

The present invention relates to electrical oscillators for radio andother purposes and methods I of operating same, more-particularly to asingle tube frequency controlled oscillator wherein the oscillatingfrequency may be varied or modulated in an easy and efficient manner inaccordance with an electrical bias potential or modulating signalapplied to the tube.

An object of the invention is therefore to provide an oscillatorutilizing a single electron discharge tube as a means both for themaintenance of sustained oscillations and to provide a controllablereactive impedance for varying-the effective oscillating frequency in apurely electrical manner by controlling a bias potential or current.

effecting said tube.

Another object is to provide a regenerative electron tube oscillatorwherein at least a portion of the total cathode current serves toprovide an adjustable reactive component governed by a separate biasingor modulating potential and fed back upon the oscillatory circuit forthe control of the oscillating frequency.

There has been in the past a wide usage of circuits embodying a separatereactance control tube capable of controlling or varying the resonantfrequency of the oscillatory or tank circuit of a high frequencygenerator in a purely electrical manner by varying a bias potentialapplied frequency responsive discriminator. Likewise, in

the field of phase or frequency modulation the problem arises ofmodulating the frequency of a self-excited oscillator or the phase of anoscillator in accordance with the amplitude changes of an audio or othermodulating signal wave.

A further use exists in the automatic stabilization of the carrier orcenter frequency of any type of radio transmitter to be held withinspecified limits with respect to an assigned value.

In these and other cases in the art, a special reactance control tube isused which in fact constitutes an amplifier excited by a quadraturepotential derived. from the currents flowing in the circuit to becontrolled and supplying an amphfied quadrature voltage which isimpressed .upon

the circuit to vary its effective or virtual reactance and in turn thefrequency or phase of the oscillations produced. The amount of reactiveor drature current impressed upon the resonant circuit is' varied bycontrolling the transconductance of the reactance tube by means of anelectric bias potential applied to a suitable control element of thetube, causing thereby a variation of the apparent reactance of theoscillatory circuit and in turn a change of the frequency or phase ofthe oscillations produced.

The present invention purports to provide a new and improved frequencycontrolled oscillator of the above general type utili'hing a singleelectron discharge tube both for the maintenance of sustainedoscillations and as a means to supply a variable reactance for thecontrol of the oscillating frequency within substantial limits inaccordance with a biasing or modulating current or potential. I

Another object of the invention is to provide an oscillator-modulatorwhich is simple in design and requires a minimum of parts and circuitelements and. accordingly is especially suited for small and portablephase or frequency modulated transmitters.

Still another object is toprovide a frequency controlled regenerativeoscillator, wherein a single electron discharge tube is used to performthe function of oscillator tuning reactance and phase shifting means toprovide a reactance efiect for the control of the oscillating frequency.

The above and further objects of my invention will become more apparentfrom the following detailed description taken with reference to theaccompanying drawings forming part of this specification and wherein:

Figure 1 is a schematic diagram of a single tube frequency modulatedoscillator embodying the principles of the invention; Figure2 is adiagram showing a modification of Figure 1; Figure 3 is a diagramshowing a further modification of an oscillator embodying the invention;Figure 4 is a vector diagram explanatory of the function of the circuitshown in Figure 3; Figures 5 and 6 are circuit diagrams. illustratingstill further modifications of the invention; Figure 7 is a vectordiagram explanatory of the'function of the circuit shown in Figure 6;and Figure 8 is .a diagram illustrative of an automatic frequencycontrol system for use in radio receiving circuits embodying theprinciples of the invention. Like reference characters identify likeparts through-' any other electrode, and separate feedback circuit meansfrom both output circuits to said oscillatory circuit. At least one ofsaid feedback circuits is caused to supply a reactive current componentimpressed upon said oscillatory circuit to determine the frequency ofthe oscillations produced. Further means are provided to control atleast the feedback current including or constituting the reactive orquadrature feedback in I matic diagram for single tube frequencycontrolled regenerative oscillator embodying the principles of theinvention. The latter comprises an electron discharge tube ID, in theexample shown of the normal pentode type provided with a cathode l lwhich may be of either the directly heated or equi-potential type, afirst control grid 12, a screen grid l3, a suppressor grid H used as asecond (frequency) control grid, and an anode or plate I5--all arrangedsubstantially in the order named with respect to the cathode. Anoscillatory tank circuit comprising an inductance l6 shunted by acapacity II, the latter having a resistance 20 connected in seriestherewith, is connected to the cathode II and the control grid [2 andsustained electrical oscillations are maintained therein by supplyingsuitable feedback voltage fromthe screen grid serving as an outputelectrode by the provision of feedback or tickler coil 2| inserted inthe screen grid circuit and arranged in inductive coupling relation withthe tank circuit inductance l6. The screen grid is connected to thepositive pole marked B+ of a suitable high tension supply source inseries with a voltage drop resistance 22 and associate ground or cathodeby-pass condensers 23 and 24 for the high frequency currents. In acircuit of this type, the control grid l2, screen grid I 3 andassociated circuits constitute a regenerative or feedback oscillator ofknown type, the details of which may be varied as will be understood bythose skilled in the art. The plate I5 is supplied with operatingvoltage from the source B+ through a load impedance, preferably aresistance 25 which serves to develop oscillating voltage therebetweenand cathode by electron coupling with the oscillator circuit associatedwith grids I2 and I3 in a manner well understood. The plate voltagedeveloped is fed back upon the tank circuit l6, I1, 20 by directlyconnecting the plate IE to the junction point between the tank circuitcondenser l1 and series resistance 20 through a blocking condenser 26.As will appear from the following, a reactive or quadrature voltage willbe developed in this manner in the oscillatory circuit, resulting in acorresponding variation of the oscillating frequency. Items 21 and 28indicate the usual self-biasing grid condenser and leak resistance toprovide a suitable grid operating bias potential for the oscillatorcircuit. amount of reactive feedback voltage is controlled by varyingthe bias of the suppressor grid M in any suitable manner such as bymeans of a potentiometer 3| connected to a suitable biasing source suchas a battery 30 providing a varying negative bias for effecting thefrequency control of the os- The cillations produced. Alternatively, thereactive feedback control may be effected in accordance with a suitablemodulating potential supplied from a microphone circuit or the like andimpressed upon grid M.

The function and operation of the circuit aforedescribed will be furtherunderstood from the following: Considering the oscillating orcirculatory current in the tank circuit l6, I1, 20 caused by an E. M. F.induced in the inductance l6, and neglecting the effect of theresistance 20 which may be of comparatively small value, thecapacitative oscillating voltage drop developed by the condenser ll andimpressed between the cathode II and grid I2 will then lag the currentin the tank circuit by .90". Hence, the oscillating current flowing inthe screen grid circuit being in phase v with the grid potential willalso lag the current in the tank circuit and, by proper choice of thewinding sense of the tickler coil 2| relative to the tank circuitinductance 16, the E. M. F. induced in the latter by the screen gridcurrent may be caused to be in phase with the tank circuit current thusovercoming the ohmic losses in the circuit and resulting in themaintenance of sustained oscillations at a frequency equal to the tuningor resonating frequency of the circuit. From the foregoing, it will beseen that the inphase relation of the feedback voltage induced in theinductance I6 by the screen grid current is due to a double 90 phaseshift, first by the condenser H and then by the mutual inductancebetween the coils 2| and 15.

Referring now to the plate output circuit, it will be seen that theoscillating voltage developed between the plate l5 and ground byelectron coupling with the oscillator circuit is 180 out of phase withthe potential on grid l2 and this voltage is directly impressed upon thetank circuit I6, I1, 20 without any additional phase shift by developingvoltage across the resistance 20 by the proper design of the stoppingcondenser 26. In other words, since only a single quadrature phase shiftis involved (i. e. by the condenser II) for the plate feedback current,the

feedback voltage developed by the resistance 20 will lead the tankcircuit current, thus representing the equivalent of an increase of theeffective or virtual inductance of the tank circuit and resulting in acorresponding variation of the oscillating frequency.

Oscillatory output energy may be derived from the oscillator in anysuitable manner such as by way of a coupling coil 32 arranged ininductive relation with the .tank circuit inductance l6 and forming partof a subsequent circuit or utilization system connected to the terminalsmarked :v-y.

Referring to the modification shown in Figure 2, the ohmic resistance inthe tank circuit has been eliminated and a separate phase shift networkcomprising a condenser 34 and resistance 35 in series is connected inparallel to the oscillatory tank circuit. In this case, the plate I5 isconnected to the junction between condenser 34 and resistance 35 throughthe blocking condenser 26. The function of this circuit is similar tothat of the circuit according to Figure l, the

2,378,245 shift by the condenser 2e, and the third phase shift by thecondenser 34. The regenerative or .in-phase feedback component isproduced in a microphone circuit connected to the frequency control gridI4, said control circuit comprising a microphone 46, a battery or othercurrent source 44, and the primary 45 of an audio frequency transformer.The grid 14 is connected to a suitable tap point of the source 44 inseries with the secondary 41 of the audio transformer and a highfrequency control grid l4 and the main screen or 1 output grid I3.Inthis manner reactions of the grid 14 upon the regenerative orin-phase" feedback current and the stability of the oscillator areminimized or eliminated, in contrast to Figure 1 wherein the grid servesto vary the distribution of the total cathode currentupon theregenerative and degenerative feedback circuits in the manner.described.

Figure 3 shows a circuit similar to Figure 2,

wherein the grid leak resistance 28 is connected between grid l2 andcathode II and forms part of the feedback path together with thefeedback condenser 26 connected between grid l2 and plate 13.

Figure 4 represents the vector diagram explanatory of the function ofFigure 3. I indicates the,current flowing in the tank circuit l6, H, V;represents the capacitative voltage developed by the condenser I1lagging the current I by 90 and being applied to the grid l2 and cathodell of the tube. The screen grid current is is in phase with voltage V:and induces a regenerative voltage er in the tank circuit serving forthe maintenance of the oscillations. Grid voltage V; also causes platevoltage. Vp displaced by 180 and developed by the preferablynon-reactive plate load.

impedance 25. Plate voltage V causes in-phase current i: to flow throughfeedback circuit 26, 28 by proper design of feedback condenser 26,resulting in voltage drop V: developed by resistance '28. Voltage V: inturn causes feedback voltage er impressed upon the tank circuit whichvoltage, as seen from the diagram, is in quadrature with the oscillatingcurrent 'I. In this manner, both regenerative and reactive feedbackvoltages are impressed upon the tank circuit resulting both in themaintenance of sustained oscillations frequency choke coil 48 in such amanner that the battery 44 serves to supply both suitable negativebiasing potential for the grid and at the same time the operatingcurrent for the microphone. The induction .coils I6, 2| and 3|v aresuitably shielded by means of a metallic screen 43 in accordance, withwell known practice, and other details like the supply of the heatingand other operating current may be according to conventional standardswell known to those skilled .in the art.

Referring to Figure 6, I have shown 'a further modification of theinvention utilizing a ,tube which serves both as a-relay or amplifierfor the maintenance of sustained oscillations and a means for supplyinga variable phase shifted or quadrature voltage injected into theoscillatory circuit for effecting a frequency control of theoscillations produced. For this purpose, the plate I5 is operated atcathode potential or at a potential negative with respect to the cathodeby the provision of a suitable biasing source such as a battery 52shunted by an adjustable potentiometer 53. In an arrangement of thistype, provided a proper adjustment of the plate biasing poten tial, anelectron current flow to the plate is substantially prevented and aconcentrated electron oscillations produced and will cause a capacitaandcontrol of the oscillating frequency in the manner understood from theabove.

Referring to Figure 5, I have shown a further modification of theinvention similar to the preceding clrcuit but including a feedback pathcomprising blocking condenser 26 in series with the primary 4| of acoupling transformer having a secondary. connected across theoscillatory tank circuit I6, I1. The inductances of the transformerprimary and secondary are of a to the tank circuit current while the E.M. F.

induced in the secondary 40 is applied in parallel relation to the tankcircuit-current a quadrature relationship will exist between the twofeedback voltages resulting in both a regenerative and quadraturefeedback for the maintenance of the sustained oscillations and controlof the oscil-' lating frequency. In Figure 5 I have shown a tive ordisplacement current to flow through'the outer plate -circuit includinga preferably nonreactive load impedance 5l. The latter causes a voltagedrop between the plate and cathode lagging. by 90 the oscillatingvoltage between grid l2 and cathode i I. This quadrature voltagegenerated by space charge coupling of the plate with the electron streamis fed back through condenser 26 upon the oscillatory circuit in such amanner as to produce a reactive feedback voltage for the control of theoscillating frequency.

Figure 7 shows the vector diagram explanatory of the operation of Figure6. The letter I again indicates the oscillatory current flowing in thetank circuit l6, II, V; represents the oscillating voltage between thegrid l2 and the cathode ll, V5 is the screen oscillating voltage, is isthe screen current and er the induced regenerative voltage in the tankcircuit serving for maintaining the oscillations in a manner similar tothat described hereinabove. As pointed out, rid voltage Vg furthercauses a plate voltage Vp produced by space charge coupling and laggingbehind the grid voltage by 90. Plate voltage Vp, by suitable design ofthe capacity of the condenser 26, drives a leading current z' throughfeedback condenser 26 and the tank circuit l6, 11, resulting in aquadrature feedback voltage being impressed upon the tank circuit. Theamount of quadrature feedback may be controlled in any suitable mannerby'varying the degree of the space charge coupling such as bycontrolling the negative bias of the plate l5 by the aid ofpotentiometer 53.

Another method of controlling the quadrature feedback or oscillatingfrequency consists in varying the screen grid potential which also willresult in a control of the space charge coupling. An arrangement of thelatter type is shown in Figure 8 in the form of an automatic frequencycontrol system for use in a superheterodyne radio receiver. Theoscillator shown is substantially similar to that of Figure 6 and servesas combined local oscillator and reactance tube at present provided inautomatic frequency control systems. The oscillating frequency isadjusted by the potential supplied froma discriminator varying both insign and magnitude in accordance with the departure of the intermediatefrequency from the center frequency of the band pass characteristic ofthe intermediate frequency amplifier of the receiver. The discriminatorshown especially suited for use in connection with the variablefrequency oscillator proposed by the invention is of the type disclosedby U. S. Patent 2,208,091 utilizing the space charge coupling within amulti-grid tube as a phase shifting means for producing a timeresponsive or discriminating potential.

The discriminator, in the example shown, comprises a multigrid tube 55having a cathode 56, a first or signal input grid 51, a screen grid 58,a second control grid which may be the suppressor of a normal pentode50, and a plate 6|. Intermediate frequency signal potential is impressedupon input grid 51 and serves to excite a tuned circuit 63 comprising aninductance and a condenser in parallel and resonating at the assignedintermediate frequency of the receiver, said tuned circuit beingconnected between the outer control grid 60 and ground or cathode andsubstantially exteriorly decoupled or shielded from the rest of thecircuit. Due to the variable space charge coupling between grid 56 andgrid 80, the circuit 63 will develop a potential at intermediatefrequency varying in phase both in sense and magnitude in accordancewith the departure of the intermediate signal frequency from itsassigned value. Due to the intermodulation effect of the grids 51 and60, there is produced in the plate circuit a direct current componentvarying substantially in proportion to the intermediate frequencydeparture from the assigned frequency and the corresponding platevoltage variations produced by means of a suitable plate load impedancesuch as a resistance 65 are utilized according to the invention to serveas screen supply voltage for the local oscillator by directly connectingthe screen grid l3 of tube into the plate 6| of tube 55 in series with asuitable filter 61 to eliminate both high frequency and modulatingfrequency components and to cause a control of the local oscillator, forthe compensation of spurious and slow carrier frequency variations as aresult of oscillator drift, heat influences and other causes. Similardirect current variations occur in the screen grid circuit of thediscriminator tube 55 but in opposite phase to the plate currentvariations, whereby, by the omission of the by-pass condenser for thescreen grid resistance 66, the screen voltage may be used for supplyingthe operating voltage for the screen grid of the oscillator I to obtaina frequency control in the proper sense forthe compensation of theintermediate frequency deviations from their assigned frequency. Item 62indicates a standard condenser-shunted resistance in the cathode returnfor the plate'current to provide suitable grid operating bias.

It will be evident from the foregoing that the invention is not limitedto the specific circuits and details shown and disclosed herein forillustration, but that the underlying novel principle will besusceptible of numerous modifications and variations coming within thebroader scope and spirit of the invention as defined in the appendedclaims. The specification and drawings are accordingly to be regarded inan illustrative rather than in a limited sense.

I claim:

1. In an oscillator, an electron discharge tube having at least acathode, a first control grid, a screen grid forming a first outputelectrode, a second control grid and a second output electrode allarranged substantially in the order named. an oscillatory circuitconnected between said first control grid and cathode, a regenerativefeedback path between said screen grid and said circuit to applyin-phase feedback energy to said circuit to generate sustainedelectrical oscillations, a further feedback path between said secondoutput electrode and said circuit including means to cause additionalfeedback energy in quadrature phase relation to said first energy to beimpressed upon said circuit, means for applying varying bias potentialto said second control grid to correspondingly vary the frequency of theoscillations produced, and further means for substantially preventingreaction between said second control electrode and said first outputelectrode.

2. In an oscillator, an electron discharge tube having at least acathode, a first control grid, a screen grid forming a first outputelectrode, a second control grid and a second output electrode allarranged substantially in the order named, an oscillatory circuitconnected between said first control grid and cathode, a regenerativefeedback path between said screen grid and said circuit to applysubstantially in-phase feedback energy to said circuit to generatesustained electrical oscillations, a further feedback path between saidsecond output electrode and said circuit including phase shifting meansto cause additional feedback energy in substantially quadrature phaserelation to said first energy to be impressed upon said circuit, meansfor apply;ng varying bias potential to said second control electrode tocontrol the amount of quadrature feedback energy supplied by said secondoutput electrode to correspondingly vary the frequency of theoscillations produced, and further means to substantially preventvariation of the in-phase feedback energy supplied by said screen gridin response to variations of said bias potential.

3. In an oscillator, an electron discharge tube having at least acathode, a first control grid, a screen grid forming a first outputelectrode, a second control grid and a second output electrode allarranged substantially in the order named, an oscillatory circuitconnected between said first control grid and cathode, a regenerativefeedback path between said first output electrode and said circuit toapply substantially in-phase feedback energy to said circuit to generatesustained electrical oscillations, a further feedback path between saidsecond output'electrode and said circuit including phase shifting meansto cause adsaid further screen grid at a high positive potential forsteady current and at substantially zero potential for the oscillatingcurrent with respect to said cathode.

4. In an oscillator, an electron discharge tube having atleast acathode, a first control grid, a screen grid, a second control grid andan anode all arranged substantially in the order named, a' paralleltuned oscillatory circuit comprising inductive and capacitativereactance elements in shunt relation, said circuit being connectedbecircuit. to generate sustained electrical oscillations, and a furtherfeedback path substantially free of phase rotating circuit elementsbetween said anode and the'junction point of. said nonreactiveimpedancetand the reactance element in series therewith.

5. In ambscillator, anelectron discharge tube having at least a cathode,a first control grid, a screen grid, a second control grid and an anodeall arranged substantiallyin thenorder named, a parallel tunedoscillatory circuit comprising inductive and capacitative reactanceelements in shunt.relat'on, said circuit being connected between saidfirst control grid and cathode, a nonreactive impedance in series withone of said reactance elements, feedback coupling means between saidscreen grid and said circuit to apply substantially in-phase feedbackenergy to said circuit to generate sustained electrical oscillations, afurther feedback path substantially free of phase rotating circuitelements between said anode and the junction point of saidnon-reactive'impedance and the reactanceelement in series therewith,means for applying a variable bias potential to said second control gridfor controlling the frequency of the oscillations produced, and means toprevent variations of the inphase feedback energy responsive to saidbias potentiaL- 6. In an oscillator, an electron discharge tubecomprising an oscillation generating control and an output electrode, anoscillatory circuit coupled to said .control electrode, regenerativefeedback means connecting said output electrode to said circuit to applyin-phase feedback energy to said circuit, whereby to generate sustainedelectrical oscillations, a further electron receiving output electrodecoupled to the oscillation generating electrodes by the electron streamof the tube,

phase shifting impedance means coupling said further output electrode tosaid oscillatory circuit for feeding thereto energy of the-generatedfrequency displaced in phase with respect to the generated frequency,and means substantially decoupled from the oscillation generatingelectrodes for varying the amount of phase-shifted feedback energysupplied by said further output electrode to thereby control thefrequency of the oscillations produced.

